With rapid development on semiconductor technology, performance requirements of transistors have been continually increased. In order to fabricate transistors with an ultrahigh frequency and an ultrahigh speed, a high electron mobility transistor (HEMT) technology has been developed. The ultrahigh frequency and the ultrahigh speed of a transistor may be achieved by increasing the carrier mobility to reduce the delay time of the signal transfer.
FIG. 1 illustrates an existing HEMT. The HEMT includes: a sapphire substrate 202; a gallium nitride (GaN) layer 204 on the sapphire substrate 202; an aluminum gallium nitride (AlGaN) layer 216 on the GaN layer 204 serving as potential energy barrier layer; a gate 210 on the AlGaN layer 216; and a source region 208 and a drain region 212 on the AlGaN layer 216 at both sides of the gate 210. The GaN layer 204 and the AlGaN 216 form a modulation doped heterojunction. A two dimensional electron gas (2-DEG) is formed between the GaN layer 204 and the AlGaN layer 216. A surface intensity of a 2-DEG may be controlled by the gate voltage Vg, thus the working current of the transistor may be controlled. Because 2-DEG may not be affected by the scattering of ionized impurities, it may has a relatively high carrier mobility.
Further, referring to FIG. 1, a fluoride ion layer 206 may be formed in the GaN layer 204 under the gate 210, a breakdown problem may be solved by locally increasing the potential energy of the GaN layer 204.
However, existing techniques may be unable to match the carrier mobility requirements of the continuous development of the semiconductor technology. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.